As planar semiconductor devices are scaling down continuously, short channel effects are becoming more and more prominent. Therefore, 3D semiconductor devices, such as FinFETs (Fin Field Effect Transistors), are proposed. In general, a FinFET includes a fin formed vertically on a substrate and a gate intersecting the fin. Therefore, a channel region is formed in the fin, and has a width mainly determined by the height of the fin. However, in integrated circuit manufacture, it is difficult to control the heights of fins formed on a wafer to be identical, resulting in performance inconsistency between devices across the wafer.